> Wrong.
>
> 1) If you don't receive "destination unreachable" packet, you know
> nothing about the target host yet. This is not perfect-network world.
> There can be other fw/router anywhere in the way, killing this type of
> icmp traffic.
>
> 2) It slows scans a lot.

Only for people too stupid for doing port scans (a rare defect even
among script kiddies).

> You can of course do scannig in parallel, but
> don't be surprised, when you find yourself killed with no mercy by IDS,
> after matching SYN threshold. 1000+ syns/sec form IP adress to monitored
> system is sure ban.

Cool. Your IDS just banned the IPs of your customers mail-, web- and
proxy-servers. Spoofing IP adresses just to mess with such automatic
systems is easy.
Regards,
Alex.
--
"Opportunity is missed by most people because it is dressed in overalls and
looks like work." -- Thomas A. Edison
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